This invention relates to a two-wire frequency division multiplex system.
A two-wire frequency division multiplex system enables the transmission of multi-channel telephone messages in both directions on the same pair of wires by using different frequency bands for the transmitting and the receiving directions. The frequency bands are divided into smaller channel bands each of which contains the modulated information of a predetermined one of the multiplexed channels respectively.
Some existing two-wire frequency division multiplex systems comprise two terminal stations A and B, connected by a two wire line, which operate on the same frequency channels (baseband) but each of the stations contains additional frequency conversion equipment connected to either the transmit or the receive line terminals. Thus, station A may have its transmission converted to a different frequency band (sub-group) to be sent over the line and converted back to the original baseband for reception by station B. The process may be reversed by reconnecting the converter sections at both the terminal stations. A common sub-group carrier is used for both conversions.
Due to the narrow choice of sub-groups, it may be difficult to achieve the up and down conversion with sufficient degree of linearity such as to avoid generation of intermodulation products which would interfere with operation of the system. The problem may be particularly acute when close harmonics of one sub-group fall inside the frequency band of the other.
Further, conventional frequency division multiplex systems suffer from higher distortion and interference in one direction than in the other because a signal from station A to station B has to pass through two conversion stages before reaching the recipient, whereas a return signal from B to A will be received directly on the baseband frequency. Thus conversion stages have to be built to exacting specifications in order to guarantee the minimum quality standard for both directions.